Method of and apparatus for testing an engine or a compressor

ABSTRACT

A method of and apparatus for testing an engine or compressor wherein one of an inlet and an exhaust valve (19) is actuated by a hydraulic piston and cylinder arrangement (11) and the flow of hydraulic fluid to the piston and cylinder arrangement (11) is controlled, provides variation of the motion of the valve (19) by adjustment of a control (14, 10) for controlling the hydraulic fluid. Preferably the control is programmed such that the motion of the valve (11) corresponds to the motion that would result from the use of a particular camshaft in the engine or compressor to control valve motion.

The invention relates to a method of testing an engine or a compressor.The invention also relates to apparatus used in the method.

The present invention will be discussed with reference to itsapplication to a reciprocating internal combustion engine. However, theinvention should not be considered limited to such an application sinceit can be equally well applied to any type of engine or compressor whichuses valves to control flow of fluid and/or gas (e.g. a reciprocatingcompressor).

The motion of valve gear at the cylinder head is normally controlled bythe rotation of one or more cam shafts. The cam shafts act to open andclose the inlet and exhaust valves of the internal combustion engine.The timing of the opening and closing of the valves is determined by theprofile of the cam shaft. Different cam shaft profiles are chosen fordifferent engines. The profile of the cam shaft has a large impact onthe efficiency and performance of an engine.

Generally different cam profiles are tested on a test bed arrangement.In each instance a cam shaft with a particular profile must be machinedand then connected in the internal combustion engine before theparticular cam profile can be tested. This is a long and costly process.The present invention provides means by which this process can be mademore cost effective and can be speeded considerably.

The present invention provides a method of testing an engine orcompressor wherein an inlet and/or exhaust valve is actuated by ahydraulic piston and cylinder arrangement and the flow of hydraulicfluid to the piston and cylinder arrangement is controlled, variation ofthe motion of the valve being achieved by adjustment of the controlmeans for controlling the hydraulic fluid.

Preferably the control means for controlling the hydraulic fluid areprogrammed such that the motion of the valve corresponds to the motionthat would result from the use of a particular camshaft in the engine orcompressor to control valve motion.

Preferably the programming of the control means is varied during testingof the engine or compressor such that the motion of the valve is variedto correspond to the motion that would result from the use of variousdifferent camshafts in the engine or compressor.

The invention also provides apparatus for testing an engine orcompressor comprising valve control apparatus for controlling valvemeans which opens and closes an inlet or exhaust port of the engine orcompressor, which valve control apparatus comprises; an actuator whichcomprises a piston within a cylinder, which piston is connected eitherdirectly or indirectly to the valve means such that motion of the pistoncauses motion of the valve means; processing means for deriving thestroke position of the engine or compressor; position measurement meansfor measuring directly or indirectly the position of the piston withinthe actuator cylinder; and control means for controlling the actuator bycontrolling the flow of fluid or gas into at least one of the chambersdefined by the surfaces of the piston and the interior surface of thecylinder; wherein the control means controls the actuator to cause thevalve means to open and close the port and wherein the control meansoperates with regard to the stroke position of the engine or compressorand in accordance with a programme of instructions provided by the userof the apparatus, which programme of instructions can be altered by theuser as required.

Preferably a programme of instructions is provided by the user such thatthe motion of the valve means corresponds to the motion that wouldresult from the use of a particular camshaft in the engine or compressorto control valve motion.

Preferably the control means for controlling the actuator compriseselectrical or electronic processing means and a control valve controlledby the processing means by the use of electrical control signals, whichcontrol valve can connect at least one of the chambers of the actuatorto a source of pressurised fluid or pressurised gas or to an exhaust forfluid or for gas.

Preferably the processing means controls the control valve by use of aclosed loop position or velocity feedback control system wherein thefeedback signal is derived from the signal produced by the displacementtransducer.

Preferably the position measuring means for measuring the position ofthe piston with respect to the actuator cylinder comprises adisplacement transducer connected to the piston which provides anelectrical signal indicative of the said relative position.

Preferably the apparatus additionally comprises processing means forproducing an electrical signal representative of the operational speedof the engine or compressor. The processing means for producing a signalrepresentative of the operational speed of the engine or compressorpreferably comprises means for differentiating the signal representativeof the stroke position of the engine or compressor. Preferably theapparatus of the invention has processing means which controls thecontrol valve having regard to the operational speed of the engine orcompressor as indicated by the electrical signal.

In one embodiment the control means of the apparatus controls the flowof fluid or gas into both of the chambers defined by the two surfaces ofthe piston and the interior surface of the cylinder.

Preferably the apparatus of the invention comprises resistance meanswhich increases the resistance offered to the flow of fluid or gas outof the actuator as the actuator piston approaches at least one end ofthe cylinder.

The invention further provides a method of testing an engine orcompressor comprising the steps of;

providing apparatus as hereinbefore described, programming the apparatuswith a programme of instructions which will cause an exhaust or inletvalve to move under the control of the actuator in a mannercorresponding to the motion that would result from the use of aparticular camshaft in the engine or compressor and operating the engineor compressor with the apparatus installed and controlling the motion ofthe valve in accordance with the programme of instructions.

Preferred embodiments of the present invention will be described withreference to the accompanying drawings in which;

FIG. 1 is a part cross-section through a portion of the apparatusincluding the hydraulic actuator and the control valve.

FIG. 2 is a schematic representation of a portion of the apparatusincluding a pump, a reservoir, a control valve, the hydraulic actuatorand the engine valve.

FIG. 3 is a schematic representation of the control system of theapparatus.

FIG. 4 is a cross-section of part of the hydraulic actuator of theapparatus.

FIG. 5 is a schematic representation of a second embodiment of theapparatus.

FIG. 6 is a cross-section of part of the hydraulic actuator of thesecond embodiment of the apparatus.

Referring to FIG. 1 the apparatus can be seen to comprise a controlvalve 10 and a hydraulic actuator 11 connected to the control valve 10by three passages 12, 13 and 14. Passages 13 and 14 are joined togetherat a point 15.

The actuator 11 comprises a piston 16 movable within a cylinder definedby the walls 17. The piston 16 is directly connected by a rod 18 to avalve (not shown in FIG. 1) which opens and closes an aperture openingonto a cylinder of an internal combustion engine.

The apparatus also includes a displacement measurement device 49, suchas an LVDT (Linear Variable Differential Transformer) or a Hall EffectTransducer, connected to the piston 16 for measurement of the relevantdisplacement of the piston 16 with respect to the cylinder 17.

The control valve 10 is a standard control valve. In the preferredembodiment in a engine with a maximum revolutionary speed of 7,000 rpmthe valve has a frequency response of 350 Hz.

The arrangement can be seen schematically in FIG. 2. The hydraulicactuator 17 is shown schematically as a cylinder comprising a piston 16connected by the rod 18 to the valve 19. The piston 16 is caused to movewith respect to the cylinder 17 by supplying fluid under pressurethrough the two pipes 15 and 12 to both sides of the piston 16.

The control valve 10 is shown schematically in FIG. 2 with two controlports connected to lines 15 and 12, a pressure port connected to line 20and a return port connected to line 25. The pressure port 20 isconnected via the line 20 to a pump 2 1 and a reservoir 22. The controlvalve 10 supplies pressurised fluid to either line 15 or 12 dependingupon an input signal received from via the line 26 from the processingmeans 40. When one line is so connected the other line is connected bythe control valve 10 to the exhaust 24 via the line 25.

An expanded cross-sectional view of part of the hydraulic actuator 11 isshown in FIG. 4. In FIG. 4 there can be seen the two tubes 13 and 14 therod 23 of the displacement measurement device 49, the piston 16 and thecylinder defined by the walls 17. Circular seals 36, 37 and 38 areprovided to prevent hydraulic fluid from passing from one compartment toanother.

The FIG. 4 shows clearly an arrangement which forms the resistance meansof one embodiment of the invention. The resistance is shownschematically at 90 in FIG. 2. The purpose and method of operation ofthe resistance means shall be hereinafter described. The resistancemeans of the first embodiment of the invention comprises an annular ringof deformable resilient material 31, an annular washer 32 and a sleeve33. The sleeve 33 surrounds the rod 23 of the displacement measurementdevice 19. The sleeve 33 has a series of apertures 34 therein whichallow communication of the inside of the sleeve with the outsidethereof. Another important integer in the resistance means is an orificeformed by a nozzle 35.

The control system of the apparatus can be seen in FIG. 3. Theprocessing means of the apparatus is contained within the dotted line40. From FIG. 3 it can be seen that there are three inputs to theprocessing means. The first of these inputs is an electrical signalcorresponding to the crank angle. The crank angle is measured at 41. Thecrank angle signal corresponds to an instantaneous measurement of theangle that the crankshaft makes with an arbitrary fixed reference. Thissignal gives an indication of the stroke position of the engine; that isto say an indication of the relative displacement of the working pistonsto the working cylinders of the engine. Many alternatives to crankshaftposition measurement could be used, for instance measurements taken fromthe timing mechanism of the engine.

The instantaneous stroke position measurement is differentiated at 42 togive the speed of revolution of the engine in revolutions per minute.This parameter forms another input into the processing means 40. Thecrank angle measurement and the RPM measurement are used not only by theprocessing means but can also used by processing means for controllingignition timing and fuel injection.

The third input to the processing means 40 is a position measurement.The displacement measurement means 49 measures the position of theactuator piston 16 with respect to the actuator cylinder formed by thewalls 17. The position measurement is input to the processing means. Thesignal is used in the closed loop control system of the processing means40, the signal providing the necessary feedback loop.

The processing means 40 has one output 43. The output is an electricalsignal which controls motion of the control valve 10. The control valve10 then controls the motion of the piston 16.

The method of operation of the processing means 40 will be hereinafterdescribed, once the physical operation of the actuator 11 has beendescribed.

The method of operation of the control apparatus will now be describedwith reference to the drawings and with reference to the precedingdescription of the drawings.

Referring firstly to FIG. 2 of the drawings, the piston 16 can movewithin the cylinder 17 and is connected via a rod 18 to a valve 19. Thevalve 19 opens and closes an aperture which opens on to a cylinder ofthe internal combustion engine. The valve can shut either an inlet portopening on to the cylinder or an exhaust port.

The piston 16 is caused to move within the cylinder 17 by applying apressure difference thereacross. The pressure difference is applied bysupplying hydraulic fluid under pressure to one side of the piston 16,whilst connecting the hydraulic fluid on the other side of the piston 16to a sink.

The control valve is used to control the motion of the piston 16. If theservo-valve 10 is used to cause the valve 19 to move towards the valveseat then the servo-valve acts to connect the line 12 to line 20. Line20 receives a supply of hydraulic fluid under pressure from a pump 21and a reservoir of pressurised hydraulic fluid 22.

Whilst the control valve 10 connects the line 12 to the line 20 tosupply hydraulic fluid under pressure to the lower side of the piston16, the valve also acts to connect line 15 to a sink 24. Sink 24 is asink for hydraulic fluid and contains fluid at low pressure. Byconnecting the line 15 to the sink 24 the servo-valve enables fluid toflow from the upper portion of the cylinder 17 through the line 15 andout to the sink 24.

The reservoir of pressurised hydraulic fluid 22 is used to maintain thesupply pressure at 20 at an approximately constant value. The pump 21could be powered by motion of the engine which varies during use of thevehicle and therefore the supply of hydraulic fluid from pump 21 canvary. The reservoir 22 contains a supply of pressurised fluid and actsto stabilise the fluctuations in supply of pressurised hydraulic fluid.

If the test apparatus of the invention is to be used in a static benchtest of an engine, then the pressurised hydraulic fluid need not besupplied for a pump powered by the motion of the engine. Instead, thepressurised fluid could be provided by a source external of the engine.Such an external source would be preferable in a static bench testarrangement, since the output parameters measured during the engine testwould more closely follow those of an engine using a particular machinecam shaft, no power being lost to the test apparatus. However, even in astatic test bench arrangement, a reservoir 22 is required. The reservoir22 helps to offset problems caused by inertia of the hydraulic fluid.When the control valve 10 opens to require the supply of pressurisedhydraulic fluid, a large amount of fluid is required quickly. Thehydraulic fluid has inherent inertia. If the supply is restricted tothat available in the supply line from the pump 21 then the inertia ofthe fluid may cause inaccuracies in control, since insufficient fluidmay be readily available for supply to the actuator. The reservoir 22offsets this difficulty by providing a ready supply of pressurisedfluid. In practice, the reservoir 22 should be situated as closely aspossible to the control valve 10.

Referring to FIG. 1 of the drawings line 12 can be seen to supply thelower half of the hydraulic actuator 11. The line 15 supplies hydraulicfluid to the upper portion of the cylinder 17 through Lines 13 and 14.

In practice the apparatus of the invention has been found to be limitedin performance by the frequency response of the control valve 10 and theposition measurement means 49. The control system is not sensitiveenough to control the decceleration of the engine valve onto the seat ofthe cylinder head. The problem is enhanced since the fluid containedwithin the hydraulic actuator is not a perfectly incompressible fluid.The fluid tends to compress under rapid decceleration of the valve. Thiscompression causes inaccuracies in control. Further inaccuracies arecaused when the fluid tries to expand, applying pressure to the piston.In effect the compressibility of the fluid within the hydraulic actuatorcauses "bouncing" of the controlled valve. In order to overcome thesedifficulties the apparatus includes a resistance means the operation ofwhich shall now be described.

The resistance means only operates during the last portion of the upwardstroke of the piston, as the valve approaches the valve seat. As thepiston 16 moves away from the engine to bring the valve 19 into contactwith its respective valve seat, the non-deformable washer 32 (FIG. 4)contacts the lower end of the sleeve 33. Before such contact hydraulicfluid may flow out of the cylinder 17 through the outlet 14 by passingwithin the sleeve 33 and through the holes 34 which communicate with anannular space defined within the actuator. When the washer contacts withthe sleeve 33 the resilient component 31 deforms to seal against the rod23 of the displacement measurement means 19. The seal prevents furtherhydraulic fluid from passing out of the cylinder 17 through the sleeve33 to the outlet tube 14. Further fluid can only be expelled from theupper portion of the cylinder 17 through the tube 13.

A nozzle 35 is positioned within the tube 13 to provide a restrictedaperture. The restricted aperture provides resistance to flow of fluidout of the cylinder 17 as the piston 16 approaches the end of its upwardstroke. This resistance effectively damps the motion of the piston 16,slowing the associated motion of the valve towards the valve seat,slowing the valve so that impact between the valve and the valve seatdoes not occur.

The resistance means also largely eliminates the problem of the valve"bouncing" on the hydraulic fluid within the actuator since the volumeof fluid enclosed between the piston 31 and the orifice defined by thenozzle 35 is greatly reduced in comparison to the volume of fluidusually subject to compressing forces. The volume of enclosed fluid mustbe kept as small as possible to avoid any difficulties caused by theresonant frequency of the enclosed fluid.

After the piston has reached the top of its upward motion and fluidunder pressure is supplied to the top half of the piston through theinlet 14 and 13 the resistance means ceases to function, since thehydraulic fluid supplied through 14 can apply pressure to the washer 32and the piston 16. Unnecessary damping of the motion of the piston 16within the cylinder 17 is therefore avoided.

An alternative arrangement of the resistance means will now be describedwith reference to FIGS. 5 and 6. In FIG. 5 there can be seen analternative embodiment of an actuator 100 for use in the apparatus. Theactuator 100 comprises a piston attached by a connecting rod 102 to aninlet or exhaust valve 19 of an engine.

The actuator 100 has one fixed end 103 and one movable end 104. Themovable end 104 is connected to a piston 105. In the preferredembodiment described the movable end 104 is formed in one component withthe piston 105. The piston 105 is movable within a second cylinder 106.A cavity 107 is defined between the surface 108 of the piston 105 andthe uppermost surface 109 of the second cylinder 106. Hydraulic fluid issupplied to the chamber 107 by a line 110. Positioned in the line 110 isan orifice 111.

The resistance means 112 of the second preferred embodiment of theinvention is shown schematically in FIG. 5 in the line 113. Line 113connects the chamber 114 of the actuator 100 to the valve 115. A line116 connects the lower chamber 117 of the actuator 100 to the valve 115.

The valve 115 is further connected by a line 118 to a source ofhydraulic fluid under pressure 119 and a reservoir of pressurisedhydraulic fluid 120. The control valve 115 is also connected by a line121 to an exhaust for fluid 122. The control valve 115 is controlled byelectrical signals received through a line 123 from the processing means40 of the apparatus.

The chamber 107 of the second cylinder 106 is connected to both thesource of pressurised fluid 119 and the reservoir of pressurised fluid120 via the line 110. The line 110 is not directly connected to thesource of pressurised fluid 119 and the reservoir of pressurised fluid120. Instead, the line 110 is connected to both the source 119 and thereservoir 120 via a regulating valve 124. The regulating valve 124regulates the pressure of the fluid in line 110 to one tenth of thesystem pressure.

In FIG. 6 there can be seen a part cross-section of the actuator 100. InFIG. 6 there can be seen the movable end 104 connected to the piston105, as well as the second cylinder 106, the chamber 107 and the supplyline 110.

The resistance means of the second embodiment of the invention comprisesthe essential integers of a frusto-conical protrusion 125 on the lowersurface of the movable end 104 and also the frusto-conical indentation126 on the upper surface of the piston 101.

In normal operation of the actuator 100, as the piston 101 approachesthe end 104 fluid flows freely out of the chamber 114 to the line 113via cavities 127 and 128 in the movable end 104. However, as the piston101 nears the movable end 104 the flow of fluid out of the actuator 100to the line 113 becomes constricted. The lower surface of the movableend 104 and the upper surface of the piston 101 define an annular cavity115. As the piston 101 moves towards the movable end 104, the flow offluid out of the region 115 becomes restricted due to the narrow natureof the annular aperture 116 defined between the protrusion 125 and theuppermost surface of the piston 101.

As the piston 101 approaches nearer and nearer the movable end 104,thegreater becomes the resistance offered to the flow of fluid out of theregion 115. This arises since the constriction formed between theuppermost piston surface and the lowermost movable end surface becomesnarrower and narrower. The progressively increasing resistance offeredto the flow of fluid out of the actuator 100 progressively damps themotion of the piston 101 towards the movable end 104. The progressivelyincreasing resistance allows the progressive deceleration of the motionof the valve 19 towards its associated valve seat. Unlike the firstembodiment of the resistance means described above, the resistance meansof the second embodiment progressively increases the resistance to theflow of fluid out of the actuator 100 and hence progressively increasesthe decceleration forces on the valve 19. In the first embodimentdescribed above, the resistance offered to the flow of fluid out of theactuator 1 1 is determined by the orifice formed in the nozzle 35.

The second embodiment of the invention relies upon the fact that verylittle or no fluid is left between the piston 101 and the movable end104 when the valve 19 is against its respective valve seat. Thereforeadjustment means are provided which allow the movable end 104 to bebrought into contact with the piston 101 when the valve 19 is againstits respective valve seat.

The adjustment means comprises the line 110, the orifice 111 and thepressure regulating valve 124. When the pressure regulating valve 124 isoperated, a supply of fluid under pressure is provided to the chamber107 of the second cylinder 106. The flow of fluid into the chamber 107is kept to a very low level by use of the orifice 111. By theintroduction of fluid into the chamber 107 the piston 105 causes the end104 to move down to mate with the piston 101.

Motion of the movable end 104 is kept to a minimum during operation ofthe actuator 100, since the nozzle 111 restricts any flow of fluid intoor out of the chamber 107. Further, the compressibility of the fluid inthe chamber 107 has negligible effects, since the piston 105 acts over acomparatively large area, whilst the volume of fluid contained withinthe chamber 107 is small.

The method of operation of the processing means 40 of the apparatusshall now be discussed, with reference to FIG. 3.

As mentioned before, signals corresponding to the crankshaft positionand to the speed of revolution of the engine are input into theprocessing means. The signals are then processed using a "look-up table"with linear interpolation, in box 45. The "look-up table" is programmedinto apparatus by the operator. The operator can in this way instructthe system to follow a particular cam shaft profile.

A position signal for the valve is input into a summing junction at 46and is summed with the actual valve position as measured by thedisplacement measuring means. The resulting error signal is fed into aPID (Proportional, Integral, Differential) control system at 47. Theoutput of the PID 47 is fed into a decision box 48. The decision boxdecides whether the control system requires the valve to move towards oraway from the valve seat. In mathematical terms the decision box decideswhether the drive signal is greater or less than zero. Depending uponwhether the drive signal is greater or less than zero the processingmeans multiplies the drive signal by a constant GF or GB. This constantconverts the position control signal into a signal controlling theservo-valve. Different scaling factors GF and GB are needed since thepiston 16 has different surface areas on either side thereof, since itis attached to the rod 18 on one side and is attached to the rod 23 onthe other side.

Whilst there has been described above a system in which the controlvalve 10 controls the flow of fluid into both chambers defined by thesurfaces of the piston 16 and cylinder 17, the applicants envisageapparatus in which the flow of fluid into one chamber need only becontrolled. If resilient means, such as a spring, are provided to biasthe piston in one direction then control of the flow of fluid into onechamber and allow the control of the motion of the piston.

Whilst the system described above uses pressurised hydraulic fluid, theapplicant envisages that compressed gas could also be used to causemotion of the piston 16. However, the use of hydraulic fluid is suitedto the use of the apparatus in an internal combustion engine sincepressurised hydraulic fluid is readily available in the form of oilpressurised by an oil pump powered by the output of the engine.

Whilst above the invention is described in its application to areciprocating internal combustion engine, it should be appreciated thatthe invention could equally well be applied to any type of engine whichuses valves to control the flow of fluid or gas therein or to any typeof compressor which uses valve gears (e.g. reciprocating compressor).

It will be appreciated from the foregoing that the invention enables anoperator to test the performance of different cam profiles when used inan engine or compressor without having to have the cam profiles machinedand connected to the engine. The process therefore speeds the testingprocess for an engine or compressor as well as reducing costs.

We claim:
 1. A method of testing an engine or compressor wherein ahydraulic piston and cylinder arrangement is provided to actuate aninlet valve, a control means is provided to control a flow of hydraulicfluid to the hydraulic piston and cylinder arrangement, and the inletvalve motion is varied by adjustment of the control means forcontrolling the hydraulic fluid, the method including the step ofprogramming the control means for controlling the flow of hydraulicfluid with a program which instructs the control means to control theinlet valve motion such that the inlet valve motion corresponds to aparticular valve motion which would result from the use of a particularcamshaft in the engine or compressor to control the inlet valve motion.2. A method of testing an engine or compressor as claimed in claim 1which further includes the step of programming the control meanssuccessively with a plurality of different programs during testing ofthe engine or compressor, the different programs instructing the controlmeans to control the valve motion such that the inlet valve motioncorresponds to a motion that would result from the use of selectedcamshafts in the engine or compressor.
 3. An apparatus for testing anengine or compressor, the apparatus comprising a valve control apparatusfor controlling a valve means which opens and closes a port of theengine or compressor, wherein the valve control apparatus comprises:anactuator which comprises a piston within a cylinder, which piston isconnected to the valve means such that the a motion of the piston causesmotion of the valve means; processing means for deriving a strokeposition of the engine or compressor; position measurement means formeasuring a position of the piston within the cylinder; control meansfor controlling the actuator by controlling a flow of fluid into atleast one chamber defined by surfaces of the piston and an interiorsurface of the cylinder; and wherein the control means controls theactuator to cause the valve means to open and close the port and whereinthe control means operates with regard to the stroke position of theengine or compressor and in accordance with a program of instructionsprovided by a user of the apparatus, which program of instructions canbe altered by the user as required.
 4. Apparatus for testing an engineor compressor as claimed in claim 3 wherein a program of instructions isprovided by the user such that the motion of the valve means correspondsto a motion that would result from the use of a particular camshaft inthe engine or compressor to control the motion of the valve means. 5.Apparatus for testing an engine or compressor as claimed in claim 3wherein the control means for controlling the actuator comprises asecond processing means and a control valve controlled by the secondprocessing means using electrical control signals, which the controlvalve can connect the chamber of the actuator to one of a source ofpressurized fluid and a fluid exhaust.
 6. Apparatus for testing anengine or compressor as claimed in claim 5 wherein the processing meanscontrols the control valve by use of a signal and one of a closed loopposition and velocity feedback control system wherein the signal isderived from the position measuring means.
 7. Apparatus for testing anengine or compressor as claimed in claim 3 wherein the positionmeasuring means for measuring the position of the piston with respect tothe actuator cylinder comprises a displacement transducer connected tothe piston which provides a first electrical signal indicative of therelative position.
 8. Apparatus for testing an engine or compressor asclaimed in claim 7 further comprising additional processing means forproducing a second electrical signal representative of the rotationalspeed of the engine.
 9. Apparatus for testing an engine or compressor asclaimed in claim 8 wherein the additional processing means for producingthe second electrical signal representative of the rotational speed ofthe engine comprises means for differentiating with respect to time asignal representative of the rotational position of the crankshaft. 10.Apparatus for testing an engine or compressor as claimed in claim 8wherein the additional processing means controls the control valvehaving regard to the rotational speed of the engine as indicated by thesecond electrical signal.
 11. Apparatus for testing an engine orcompressor as claimed in claim 3 wherein the control means controls theflow of fluid into two chambers.
 12. Apparatus for testing an engine orcompressor as claimed in claim 3 wherein a resistance means is providedwhich resistance means increases the resistance offered to a flow offluid out of the actuator as the actuator piston approaches at least oneend of the cylinder.
 13. A method of testing an engine or compressorcomprising the steps of:providing an apparatus for testing an engine orcompressor including a valve control apparatus for controlling a valvemeans which opens and closes a port of the engine or compressor, whereinthe valve control apparatus comprises an actuator which comprises apiston within a cylinder, which piston is connected to the valve meanssuch that a motion of the piston causes motion of the valve means;processing means for deriving a stroke position of the engine orcompressor; position measurement means for measuring a position of thepiston within the cylinder; control means for controlling the actuatorby controlling a flow of fluid into at least one chamber defined bysurfaces of the piston and an interior surface of the cylinder, whereinthe control means controls the actuator to cause the valve means to openand close the port of the engine or compressor, and wherein the controlmeans operates with regard to the stroke position of the engine orcompressor; programming the apparatus with a program of instructions forcausing the valve means to move under the control of the actuator in amanner corresponding to a motion that would result from use of aparticular camshaft in the engine or compressor; and operating theengine or compressor with the apparatus installed and controlling themotion of the valve means in accordance with the program ofinstructions.
 14. A method of testing an engine or compressor wherein ahydraulic piston and cylinder arrangement is provided to actuate anexhaust valve, a control means is provided to control a flow ofhydraulic fluid to the hydraulic piston and cylinder arrangement and theexhaust valve motion is varied by adjustment of the control means forcontrolling the hydraulic fluid, the method including the step ofprogramming the control means for controlling the flow of hydraulicfluid with a program which instructs the control means to control motionof the exhaust valve such that the exhaust valve motion corresponds to aparticular valve motion which would result from the use of a particularcamshaft in the engine or compressor to control the exhaust valvemotion.
 15. A method or testing an engine or compressor as claimed inclaim 14 which further includes the step of programming the controlmeans successively with a plurality of different programs during testingof the engine or compressor, the different programs instructing thecontrol means to control the exhaust valve motion such that the exhaustvalve motion corresponds to a motion that would result from the use ofrespective different camshafts in the engine or compressor.